Differential diaphragm carburetor

ABSTRACT

A differential diaphragm carburetor for an internal combustion engine comprising two diaphragms of different size interlocked with each other and one of said diaphragm interlocking with a valve which controls inflow of fuel to the carburetor from a fuel supply system, whereby the negative pressure in the fuel mixing chamber is applied to either one or both of the diaphragms and the operation of the valve is regulated by the difference in the size of the two diaphragms.

v United States Patent 11 1 1111 3,779,529 Kimura Dec. 18, 1973 1 DIFFERENTIAL DIAPHRAGM [56] References Cited CARBURETOR UNITED STATES PATENTS [75] lnventor: Ryuichi Kimura, Odawarahi, 2,068,939 1/1937 Viel 1. 261/DlG. 68

J an 2,144,017 1/1939 261/1310v 68 i 3,003,754 l0/l96l 26l/DIG. 68 I [73] Ass1gnee: Mlkunl Kogyo K.K., Ch1yoda-ku, 3,009,794 11/1961 26l/DIG. 68 Tokyo, Japan 3,201,096 8/l965 Barr 26l/DlG. 68 [22] Filed: 1972 Primary EJ camineF-Tim R. Miles [2]] Appl. No.: 233,988 Attorney-Richard K. Stevens et al.

130 Foreign Application Priority Data [57] ABSTRACT Au 26 I971 13 an 46/64770 A differential diaphragm carburetor for an internal 00530 197' Japan 46/85964 combustion engine comprising two diaphragms of dif- 197] "460265'3 ferent size interlocked with each other and one of said 1971 diaphragm interlocking with a valve which controls v v p inflow of fuel to the carburetor from a fuel supply sys- 4 1 tem, whereby the negative pressure in the fuel mixing '5. 261/35 261/DIG' chamber is applied to either one or both of the dia- [581 Fieid 68 69 A phragms and the operation of the valve is regulated by 5 the difference in the size of the two diaphragms.

22 Claims, 7 Drawing Figures PATENTEU DEC 1 8 I975 SHEET 20? 4 PATENTEU DH. 18 ms SHEET 3 BF 4 DIFFERENTIAL DIAPHRAGM CARBURETOR BACKGROUND OF THE INVENTION This invention relates to a carburetor for internal combustion engines. More particularly, this invention relates to a novel carburetor which can be easily adjusted so as to exhibit any desired fuel feed characteristics accommodated to any engine.

The rate of fuel feed (introduction of fuel into the mixing chamber or intake manifold) is determined by the difference between the fuel feed pressure conventionally, the head of fuel (mm Aq) in the fuel feed system which is referred to as primary pressure hereinafter, and the pressure (negative pressure) in the fuel mixing chamber, which is referred to as secondary pressure hereinafter. The pressure in the mixing chamber, which varies in a range as the engine is operated, is specific to the design of an engine. Therefore, in order to achieve desired fuel feed ratio, an appropriate difference between the primary and secondary pressures must be achieved. In the prior art, the fuel head of a carburetor is specific to the individual carburetor, and cannot easily be changed.

This invention provides a novel carburetor in which the fuel feed pressure can easily be varied so as to be adjusted to the engines of any design.

SUMMARY OF THE INVENTION According to this invention, a novel carburetor is provided which comprises a negative pressure chamber which communicates with a nozzle means for introducing fuel into afuel mixing chamber; a fuel-pressureregulating chamber which communicates with the negative pressure chamber by way of a passage having'a fuel jet orifice provided therein; a valve which controls inflowof fuel into the fuel-pressure-regulating chamber; whereby at least one wall of said negative pressure chamber and at least one wall of said fuel-pressureregulating chamber consist of respectively a negative pressure diaphragm and a fuel-pressure-regulating diaphragm; said two diaphragms being interlocked with each other by means of a movement-imparting means; said valve being interlocked with said fuel pressure regulating diaphragm; and the difference in effective areas of the two diaphragms determines the fuel feed pressure on the upstream side of said fuel jet orifice.

The negative pressure generated in the fuel mixing chamber is applied to the negative pressure diaphragm or both of the negative pressure diaphragm and the fuel-pressure-regulating'diaphragm and a displacement or movement, the direction of which is determined by that of the larger diaphragm and the degree of which is determined by the difference of the effective areas of the two diaphragms, is generated, and the displacement is imparted to the valve controlling inflow of fuel, and thus the fuel'feed to the engine is regulated more sensitively and more delicately than the carburetors of the prior art.

The primary pressure is adjusted bychanging effective area of one or both of the diaphragms. The change indiaphragm size can quite easily be effected by employing diaphragm-supporting plates of annular shape explained in detail hereinafter.

The carburetor of this invention is characterized in that the primary pressure or the fuel'pressure on the upstream side of the fuel jet orifice can easily be varied,

and in that the structure of the fuel-mixing chamber is made simpler than in the conventional carburetors.

In the carburetor of this invention, contraction is not provided at the fuel feed nozzle and the negative pressure in the fuel mixing chamber instantly transmitted to the fuel feed system.

BRIEF EXPLANATION OF THE ATTACHED DRAWINGS FIG. I shows a characteristic curve representing the relation between the fuel feed pressure and the quantity of fuel to be supplied to the engine.

FIG. 2. shows characteristic curves representing the relation between the size of the fuel jet orifice and the quantity of fuel to be supplied for 3 different sizes of fuel jet orifice.

FIGS. 3 through 7 are sectional views of carburetors representing four embodiments of this invention.

DETAILED DESCRIPTION OF THE INVENTION Introduction of the fuel from a fuel supply system to an internal combustion engine is effected by virture of the difference between the primary pressure (P) applied to the fuel jet orifice, that is, conventionally the head from the fuel level in the fuel (float) chamber which feeds the fuel, and the secondary pressure (p) in the mixing chamber, that is, P p. The secondary pressure p varies over a range as the engine is operated,

while the primary pressure P is constant, at a value specific to the individual carburetor.

FIG. 1 shows a typical curve (Curve A) for fuel feed characteristics of a carburetor of an engine operated at full load. The ordinate stands for quantity of fuel fed Q and the abscissa stands for the above-mentioned pressure difference P p. As seen in the diagram, the change in fuel feed quantity is not linearly proportional to the pressure difference P p.

Point a stands for a primary pressure I and a secondary pressure p,, and point a, stands for the primary pressure P, and a secondary pressure p This means that the engine is operated between a secondary pressure p, for low speed operation and another secondary pressure p for high speed operation at a constant primary pressure P Point a stands for another primary pressure P and the secondary pressure p and point a stands for the primary pressure P and the secondary pressure p And this means that the engine is operated between the secondary pressures p and p at the raised primary pressure P and the increase in primary pressure changes not only the absolute amount of fuel to be fed but also the fuel feed characteristics.

The absolute amount of the fuel to be supplied can be changed by changing the size of the fuel jet orifice as shown in FIG. 2.. In FIG. 2, the ordinate and the abscissa stand for quantity of the fuel supplied Q and the pressure difference P p respectively as in FIG. I. FIG. 2 shows fuel feed characteristics curves for an engine when fuel jet orifices of different sizes are used. Curve B stands for a carburetor having a fuel jet orifice of a larger size, Curve D stands for the same curburetor having a fuel jet orifice of a smaller size and Curve C for the same curburetor having a fuel jet orifice of a size between B and D.

When the engine is operated at full load with a carburetor the fuel feed characteristics of which are represented by Curve C, if the primary pressure is P and the secondary pressure is in the range of p and p the fuel feed characteristics then are represented by the c portion of Curve C, and the fuel feed amount corresponding to Q, Q, Suppose the amount Q for low speed operation is adequate while the fuel feed for high speed operation Q, is insufficient. In this case, if a larger orifice, say, an orifice, the fuel feed characteristics for which are represented by Curve B, is employed, the fuel supply under this condition is represented by the b b, portion of Curve B. This means that fuel feed at high speed operation is increased, but at the same time the fuel supply at low speed operation is unduly increased, too. In this case, however, if the primary pressure P is reduced, say, to P the engine will be operated with the characteristics represented by the b b portion of Curve B. Under this condition, desired proper fuel feed Q, for low speed and Q for high speed are effected. I

In the carburetor of conventional design, the primary pressure is kept constant and adjustement is made only by alteration of the jet orifice. This invention provides a novel carburetor in which the primary pressure is easily altered, and characteristics of which can optionally be accomodated to an engine with any operation characteristics.

In FIG. 3 is shown, a differential diaphragm carburetor of this invention comprising a negative pressure chamber composed of two diaphragms a negative pressure diaphragm and a fuel-pressure-regulating diaphragm to which a poppet valve for controlling the passage of fuel is attached. Member 1 is a fuel mixing chamber, 2 is a venturi of said chamber, 3 is a fuel feed nozzle, 4 is a passage for fuel, 5 is a negative pressure chamber, 6 is a negative pressure diaphragm, and 7 is an atmospheric pressure chamber provided in a carburetor body or casing 8 communicating with the atmosphere. The diaphragm 6 is secured by the casing per se and an annular plate 9. Annular plates of various sizes are prepared, and the effective area of the diaphragm is altered by selecting an annular plate of a suitable size. Member 10 is a fuel-pressure-regulating diaphragm, the effective area of which is smaller than the negative pressure diaphragm is this case. The effective area can be also changed by employment of an annular plate (not shown) as used for the diaphragm 6.

Member 1 l is a helicoidal spring which interlocks the negative pressure diaphragm and the fuel-pressureregulating diaphragm, 12 is a fuel-pressure-regulating chamber, 13 is a fuel canal connecting the fuelpressure-regulating chamber and the negative-pressure chamber, 14 is a fuel jet orifice, 15 is a valve chamber, 16 is a passage for fuel connecting the valve chamber 15 and the fuel-pressure-regulating chamber 12, which is opened or closed by the poppet valve 17 placed in the valve chamber 15 and supported by a spring 19 and attached to the diaphragm 10 by means of a needle 18. Member 20 is a fuel duct communicating with a fuel pump or'tank. When the engine is not in operation, the valve 17 closes the passage 16 by the force of the spring 19.

When the engine is started, a negative pressure p is generated in the venturi 2 and a negative pressure almost the same as the former is applied to the negative pressure chamber 5. Thus the pressure on the downstream side of the fuel jet orifice 14 becomes almost equal to p, too. The effective area of the negative pressure diaphragm 6 is larger than that of the fuelpressure-regulating diaphragm l0, and therefore the negative pressure diaphragm will press down or displace the fuel-pressure-regulating diaphragm, and the displacement corresponds to or is in proportion to the 5 area difference. The displacement of the fuel-pressureregulating diaphragm opens the valve 19 letting the fuel into the fuel-pressure-regulating chamber 12. That is to say, the fuel pressure in the fuel-pressure-regulating chamber, that is, primary pressure P on the upstream 10 side of the fuel jet orifice 14 responds to displacement of the fuel-pressure-regulating diaphragm. Therefore, when the area difference between the fuel-pressureregulating diaphragm 10 and the negative pressure diaphragm 6 is varied, the primary pressure P is varied, l5 and thus the amount of the fuel which is drawn in through the jet orifice 14 and is sprayed through the nozzle 3 via negative pressure chamber 5 is varied. In

the embodiment of this invention represented by FIG.

3, the effective area of the negative pressure diaphragm 20 is varied by selecting an annular plate 9 of a suitable size. By increasing the effective area of the negative pressure diaphragm, the fuel feed characteristics, for instance, as represented by :1 a, portion of Curve A in FIG. 1. is shifted to a a, portion of the same curve.

In this case, however, when the poppet valve 17 just begins to open or has been opened only slightly, the flow of the fuel may push back the valve to close the passage 18, and therefore the poppet valve 17 does not always sensitively respond to the delicate movement or distorsion of the diaphragm 10. In order to secure accurate working of the poppet valve 17, the atmospheric pressure chamber 7 has a cove 21, where the chamber is contiguous to a fuel chamber 23 separated by a third diaphragm 22. The fuel chamber 23 communicates with the valve chamber 15 by way of a canal 24. The third diaphragm 22 is interlocked with the negative pressure diaphragm 6 by means of a spring 25. In the gulf 22, an annular wall 26 is provided in order to prevent excessive distorsion or displacement of the diaphragm 22.

In this structure, fuel that enters the 'valve chamber 15 also passes into the fuel chamber 23. The pressure of the fuel in the fuel chamber 23 is imparted to the valve 17 by way of springs 25 and 11. And the force applied to the poppet valve 17 can be regulated to cancel the dynamic pressure applied to the valve so as to close the passage 16 by suitably selecting the effective area of the third diaphragm 22, the height of the annular wall 26, and the strength of the spring 25. Thus the working of the poppet valve 17 is made sensitively responsive to the displacement or distorsion of the diaphragm 10 even when the opening of the passage 16 is very small or pressure of a fuel pump is altered.

The above-explained mechanism of fuel chamber and the third diaphragm is advantageously applied to a carburetor of this invention which has a poppet valve which closes the fuel passage connecting the fuel pressure regulating chamber and the fuel supply system from the side of the fuel supply system.

FIG. 4 represents a second embodiment of this invention. This embodiment is characterized in that the poppet valve is not directly connected to the pressureregulating diaphragm but is interlocked with it by means of a lever. This mechanism makes possible a more sensitive and more delicate accommodation.

In FIG. 4, parts or members are the same as or correspond to the parts or members represented by the same number in FIG. 3. Fuel is introduced through the fuel duct into the valve chamber 15 and then into the fuel-pressure-regulating chamber 12 by waY of the passage 16 which is controlled by the poppet valve 17. The fuel-pressure-regulating chamber is composed of the fuel-pressure-regulating diaphragm 10 and the negative pressure diaphragm 6 and it communicates with the negative pressure chamber 5 via an jet orifice 14 as shown in the figure. The communication between the fuel-pressure-regulating chamber 12 and the negative pressure chamber 5 can be achieved by a canal 13 with a jet orifice 14' as shown in broken line, too. The fuel which is brought into the negative pressure chamber 5 is sprayed from the nozzle 3. In this embodiment, a lever 31, which is supported by a fulcrum 32 and is connected to the fuel-pressure-regulating diaphragm and the poppet valve 17 at the ends thereof, is provided in the fuel pressure-regulating chamber 12. The end connected with the poppet valve 17 is pressed by a spring 19 so as to close the fuel passage 16 when the engine is not. in operation.

' This carburetor works substantially in the same way as the embodiment represented by FIG. 3. When a negative pressure is generated in the venturi 2, the negative pressure is applied to the negative pressure chamber 5 through the nozzle 3 and draws up the negative pressure diaphragm 6. This displacement of the diaphragm 6 raises the fuel-pressure-regulating diaphragm 10 and opens the passage 16. Thus the fuel flows into the fuelpressure-regulating chamber 12 and pressurizes the chamber. In this case, the effective area of the diaphragm 10 is larger that that of the diaphragm 6.

Therefore, the pressure of the fuel introducedvin thev chamber works so as to close the passage 16, that is, to control the inflow of the fuel. Therefore, the pressure in the fuel-pressure-regulating chamber is kept rather low and the fuel supply to the engine is maintained rather high in high speed operation and rather low in low speed operation, which corresponds to the a a, portion of Curve A in FIG. 1.

On the contrary, if a fuel-pressure-regulating diaphragm smaller'than the negative pressure diaphragm is used, the primary pressure is kept rather high and the fuel supply characteristics in this case corresponds to 0 a portion of Curve A.

FIG. 5 stands for a third embodiment of this invention. In FIG. 5, parts and members are the same as or correspond to the parts or member represented by the same number in FIG. 3, too. In this embodiment, the two diaphragms are arranged in a plane, and the negative pressure chamber 5 and the fuel-pressureregulating chamber 12 are placed on one side, that is, on the upper side of the plane, and the other side or the lower side constitutes an atmospheric pressure chamber 7. And the negative pressure chamber 5 and the fuel-pressure-regulating chamber 12 communicate by a canal 13 in which a fuel jet orifice is provided. In the atmospheric pressure chamber a lever 32 is provided. The lever 32 is supported by a fulcrum 31 and is connected to the negative pressure diaphragm 6 by means of a needle 33 thereof at the free end thereof and to the fuel pressure regulating diaphragm'by means of a pressing spring 34 at a pointbetween the free end and the fulcrum. The poppet valve is supported by a spring 19 so that the passage 16 is closed when the engine is not in operation in the same way as in the'first embodiment. The valve 17 is connected to.the fuelpressureregulating diaphragm by means of a needle 18. When a negative pressure is applied to the negative pressure chamber, the negative pressure diaphragm 6 is raised and therefore the fuel-pressure-regulating diaphragm is raised so as to open the passage 16. Therefore, if a larger negative pressure diaphragm is employed, the displacement of the diaphragm 10 becomes greater and thus the pressure inside the fuel pressure regulating chamber is enhanced. In this case, the fuel supply characteristics correspond to Curve a 2 a in FIG. 1 as a general tendency.

In this embodiment, it is apparent from the drawing that control of the fuel pressure in the fuel-pressureregulating chamber 12 can be modified by changing the arm length ratio of the lever 32.

In the carburetor of FIG. 5 the poppet valve closes the fuel passage connecting the fuel-pressureregulating chamber and the fuel supply vsystem from the fuel supply system side. Therefore, the poppet valve may be pressed by dynamic pressure of the inflowing fuel and may not work sensitively responding the displacement of the negative-pressure diaphragm. This disadvantage can be eliminated by employing the mechanism of the fuel chamber and the third diaphragm explained in connection with the embodiment v of FIG. 3 as shown in FIG. 6. As seen in this drawing, a fuel chamber 23 with a third diaphragm is provided under the atmospheric pressure chamber 7, and connected with the fuel duct 20 by means of a canal 24. The third diaphragm is interlocked .with the lever means provided in the atmospheric pressure chamber 7 by means of a resilient means 25.

By this arrangement, the dynamic pressure applied to the poppet valve is simultaneously applied to the third diaphragm through the canal 24. The displacement of the third diaphragm is imparted to the fuel-pressureregulating diaphragm so as to cancel the dynamic pressure applied to the poppet valve.

The invention has been described particularly in reference to a few specific embodiments. These embodiments relate to carburetors having butterfly throttle valve. However, the technical idea of the differential diaphragmcarburetor of this invention can be applied to a throttle valve carburetor of any other type.

FIG. 7 shows an embodiment in which this invention is applied to a piston type throttle valve carburetor. In this drawing, the numbers represent parts and members the same as or corresponding to those represented by the same numbers in FIGS. 3 and 5. The basic construction of the carburetor of this embodiment is the same as the carburetor of FIG. 5 except that the negative pressure chamber 5 and the fuel-pressureregulating chamber 12 are placed on the opposite sides ofthe atmospheric pressure chamber. Member 41' is a piston type throttle valve, 42 is a fuel supply control needle valve, the upper end of which is freely supported by the throttle valve. A carburetor of this type has a long nozzle 3 in which the needle 42 moves up and down. Therefore, the negative pressure diaphragm (larger diaphragm) 6 is of an annular shape and a cylinwhich communicates with a nozzle for introducing fuel to a fuel mixing chamber; a fuel pressure regulating chamber which communicates with the negative pressure chamber by way of a passage having a fuel jet orifice; a valve which controls inflow of fuel into the fuelpressure-regulating chamber; whereby at least one wall of said negative pressure chamber and at least one wall of said fuel-pressure-regulating chamber consist of respectively a negative pressure diaphragm and a fuelpressure-regulating diaphragm; said two diaphragms being interlocked with each other by means of a movement-transmitting means; said valve being interlocked with said fuel pressure regulating diaphragm; and said two diaphragms have different effective areas so as to create a desired fuel pressure on the upstream side of said fuel jet orifice.

2. A differential diaphragm carburetor as described in claim 1, wherein said negative pressure chamber is contiguous to the fuel-pressure-regulating chamber separated by the negative pressure diaphragm; the fuel pressure regulating chamber is contiguous to an atmospheric chamber separated by the fuel pressure regulating diaphragm; the fuel pressure regulating chamber communicates with a fuel supply system, and the fuelpressure-regulating diaphragm is interlocked by means of a lever means with a valve controlling the passage between the fuel-pressure-regulating chamber and the fuel supply system.

3. A differential diaphragm carburetor as described in claim 2, wherein the diaphragms are supported by annular plates of sizes suitable to create a desired area difference.

4. A differential diaphragm carburetor as described in claim 3, wherein the negative pressure diaphragm and the fuel pressure regulating diaphragm are interlocked by a resilient member.

5. A differential diaphragm carburetor as described in claim 4, wherein said valve is a poppet valve placed in the passage connecting'the fuel-pressure-regulating chamber and the fuel supply system, said poppet valve being supported by a resilient member so as to close the passage when the engine is not in operation, and a lever supported by a fulcrum which is provided in the fuelpressure-regulating chamber and one end of which is connected with the poppet valve and the other end is connected to the fuel-pressure-regulating diaphragm.

6. A differential diaphragm carburetor as described in claim 1, wherein the negative pressure chamber is contiguous to an atmospheric pressure chamber separated by the negative pressure diaphragm; the fuel pressure regulating chamber is contiguous to the atmospheric pressure chamber separated by the fuelpressure-regulating diaphragm; and the negative pressure diaphragm and the fuel-pressure-regulating diaphragm are interlocked by a lever means.

7. A differential diaphragm carburetor as described in claim 6, wherein the diaphragms are supported by annular plates of sizes suitable to create a desired area difference.

8. A differential diaphragm carburetor as described in claim 7, wherein the fuel-pressure-regulating diaphragm and the lever means are connected with a resilient means.

9. A differential diaphragm carburetor as described in claim 8 wherein the valve controlling inflow of fuel is a poppet valve which closes the fuel passage, in

which it is placed, from the fuel supply system side, being pressed by a resilient means.

10. A differential diaphragm carburetor as described in claim 9, wherein a fuel chamber is provided at a posi tion opposite to the negative pressure diaphragm, whereby said fuel chamber communicates with the fuel supply system, is provided with a third diaphragm as the partition opposing to the negative diaphragm, and the third diaphragm is interlocked with the lever means.

11. A differential diaphragm carburetor as described in claim 10, wherein said interlocking is effected by a resilient means.

12. A differential diaphragm carburetor as described in claim 1, wherein the negative pressure chamber is contiguous to the fuel pressure regulating chamber; and the atmospheric pressure chamber respectively separated by the fuel-pressure-regulating diaphragm and the negative pressure diaphragm.

13. A differential diaphragm carburetor as described in claim 12, wherein the diaphragms are supported by annular plates of sizes suitable to create a desired area difference.

14. A differential diaphragm carburetor as described in claim 13, wherein the diaphragms are interlocked with a resilient member.

15. A differential diaphragm carburetor as described in claim 14, wherein the valve is a poppet valve which closes the passage connecting the fuel pressure regulating chamber and the fuel supply system from the fuel supply system side being pressed by a resilient means.

16. A differential diaphragm carburetor as described in claim 15, wherein a fuel chamber is provided contiguous to the atmospheric pressure chamber on the opposite side of the negative pressure chamber separated by a third diaphragm, whereby the fuel chamber communicates with a valve chamber which communicates with the fuel-pressure-regulating chamber and the fuel supply system; and the third diaphragm is interlocked with the negative pressure diaphragm.

17. A differential diaphragm carburetor as described in claim 6, wherein the negative pressure chamber and the fuel-pressure-regulating chamber are placed on opposite sides of said atmospheric pressure chamber.

18. A differential diaphragm carburetor as described in claim 17, wherein the diaphragms are supported by annular plates of sizes suitable to create a desired area difference.

19. A differential di'aphragm'carburetor as described in claim 18, wherein the valve controlling inflow of fuel is a poppet valve which closes the fuel passage in which it is placed from the fuel supply system side being pressed by a resilient means.

20. A differential diaphragm carburetor as 'described in claim 17, wherein the negative pressure diaphragm is of annular shape and is provided with a cylindrical means which contains an elongated nozzle of the needle valve receiving type.

21. A differential diaphragm carburetor as described in claim 20, wherein the diaphragms are supported by annular plates of sizes suitable to create a desired area difference.

22. A differential diaphragm carburetor as described in claim 21, wherein the valve controlling inflow of fuel is a poppet valve which closes the fuel passage in which it is placed from the fuel supply system side being pressed by a resilient means. 

1. A differential diaphragm carburetor for an internal combustion engine comprising a negative chamber which communicates with a nozzle for introducing fuel to a fuel mixing chamber; a fuel pressure regulating chamber which communicates with the negative pressure chamber by way of a passage having a fuel jet orifice; a valve which controls inflow of fuel into the fuel-pressure-regulating chamber; whereby at least one wall of said negative pressure chamber and at least one wall of said fuel-pressure-regulating chamber consist of respectively a negative pressure diaphragm and a fuel-pressure-regulating diaphragm; said two diaphragms being interlocked with each other by means of a movement-transmitting means; said valve being interlocked with said fuel pressure regulating diaphragm; and said two diaphragms have different effective areas so as to create a desired fuel pressure on the upstream side of said fuel jet orifice.
 2. A differential diaphragm carburetor as described in claim 1, wherein said negative pressure chamber is contiguous to the fuel-pressure-regulating chamber separated by the negative pressure diaphragm; the fuel pressure regulating chamber is contiguous to an atmospheric chamber separated by the fuel pressure regulating diaphragm; the fuel pressure regulating chamber communicates with a fuel supply system, and the fuel-pressure-regulating diaphragm is interlocked by means of a lever means with a valve controlling the passage between the fuel-pressure-regulating chamber and the fuel supply system.
 3. A differential diaphragm carburetor as described in claim 2, wherein the diaphragms are supported by annular plates of sizes suitable to create a desired area difference.
 4. A differential diaphragm carburetor as described in claim 3, wherein the negative pressure diaPhragm and the fuel pressure regulating diaphragm are interlocked by a resilient member.
 5. A differential diaphragm carburetor as described in claim 4, wherein said valve is a poppet valve placed in the passage connecting the fuel-pressure-regulating chamber and the fuel supply system, said poppet valve being supported by a resilient member so as to close the passage when the engine is not in operation, and a lever supported by a fulcrum which is provided in the fuel-pressure-regulating chamber and one end of which is connected with the poppet valve and the other end is connected to the fuel-pressure-regulating diaphragm.
 6. A differential diaphragm carburetor as described in claim 1, wherein the negative pressure chamber is contiguous to an atmospheric pressure chamber separated by the negative pressure diaphragm; the fuel pressure regulating chamber is contiguous to the atmospheric pressure chamber separated by the fuel-pressure-regulating diaphragm; and the negative pressure diaphragm and the fuel-pressure-regulating diaphragm are interlocked by a lever means.
 7. A differential diaphragm carburetor as described in claim 6, wherein the diaphragms are supported by annular plates of sizes suitable to create a desired area difference.
 8. A differential diaphragm carburetor as described in claim 7, wherein the fuel-pressure-regulating diaphragm and the lever means are connected with a resilient means.
 9. A differential diaphragm carburetor as described in claim 8 wherein the valve controlling inflow of fuel is a poppet valve which closes the fuel passage, in which it is placed, from the fuel supply system side, being pressed by a resilient means.
 10. A differential diaphragm carburetor as described in claim 9, wherein a fuel chamber is provided at a position opposite to the negative pressure diaphragm, whereby said fuel chamber communicates with the fuel supply system, is provided with a third diaphragm as the partition opposing to the negative diaphragm, and the third diaphragm is interlocked with the lever means.
 11. A differential diaphragm carburetor as described in claim 10, wherein said interlocking is effected by a resilient means.
 12. A differential diaphragm carburetor as described in claim 1, wherein the negative pressure chamber is contiguous to the fuel pressure regulating chamber; and the atmospheric pressure chamber respectively separated by the fuel-pressure-regulating diaphragm and the negative pressure diaphragm.
 13. A differential diaphragm carburetor as described in claim 12, wherein the diaphragms are supported by annular plates of sizes suitable to create a desired area difference.
 14. A differential diaphragm carburetor as described in claim 13, wherein the diaphragms are interlocked with a resilient member.
 15. A differential diaphragm carburetor as described in claim 14, wherein the valve is a poppet valve which closes the passage connecting the fuel pressure regulating chamber and the fuel supply system from the fuel supply system side being pressed by a resilient means.
 16. A differential diaphragm carburetor as described in claim 15, wherein a fuel chamber is provided contiguous to the atmospheric pressure chamber on the opposite side of the negative pressure chamber separated by a third diaphragm, whereby the fuel chamber communicates with a valve chamber which communicates with the fuel-pressure-regulating chamber and the fuel supply system; and the third diaphragm is interlocked with the negative pressure diaphragm.
 17. A differential diaphragm carburetor as described in claim 6, wherein the negative pressure chamber and the fuel-pressure-regulating chamber are placed on opposite sides of said atmospheric pressure chamber.
 18. A differential diaphragm carburetor as described in claim 17, wherein the diaphragms are supported by annular plates of sizes suitable to create a desired area difference.
 19. A differential diaphragm carburetor as described in claim 18, wherein the valve controlling inflow of fuEl is a poppet valve which closes the fuel passage in which it is placed from the fuel supply system side being pressed by a resilient means.
 20. A differential diaphragm carburetor as described in claim 17, wherein the negative pressure diaphragm is of annular shape and is provided with a cylindrical means which contains an elongated nozzle of the needle valve receiving type.
 21. A differential diaphragm carburetor as described in claim 20, wherein the diaphragms are supported by annular plates of sizes suitable to create a desired area difference.
 22. A differential diaphragm carburetor as described in claim 21, wherein the valve controlling inflow of fuel is a poppet valve which closes the fuel passage in which it is placed from the fuel supply system side being pressed by a resilient means. 